An exhaust gas of automobiles which use gasoline for fuel contains hazardous components such as hydrocarbon (THC), carbon monoxide (CO), and nitrogen oxide (NOx). Therefore, it is necessary to purify each of the hazardous components in such a manner that the hydrocarbon (THC) is converted into water and carbon dioxide by oxidation; the carbon monoxide (CO) is converted into the carbon dioxide by oxidation; and the nitrogen oxide (NOx) is converted into nitrogen by reduction.
As a catalyst (hereinafter, referred to as an “exhaust gas purification catalyst”) adapted to treat these exhaust gases, three way catalysts (TWC) capable of oxidizing and reducing CO, THC, and NOx have been used.
Three way catalysts are known, in which a precious metal is supported on a refractory oxide porous material having a high-specific surface area, for example, an alumina porous material having a high-specific surface area and the precious metal is supported on a substrate, for example, a monolithic substrate made of a refractory ceramic or metallic honeycomb structure or on refractory particles.
On the other hand, the exhaust gas discharged from the diesel engine contains sulfate salts based on sulfur content in a fuel, tar-like particulate matters (referred to as “PM”) derived from incomplete combustion, nitrogen oxide (NOx) or the like.
As an apparatus for removing the PM contained in the exhaust gas discharged from the diesel engine, an exhaust gas purification apparatus, which collects the PM in a diesel particulate filter (referred to as a “DPF”) and burns the collected PM at an appropriate timing to remove it, has been known.
Usually, this DPF is configured such that a porous filter substrate with a honeycomb structure forms a skeleton to collect the PM in a surface of a partition wall of the substrate when the exhaust gas flows inside the partition wall.
In both of a catalyst for purifying the exhaust gas discharged from the gasoline engine and a catalyst for purifying the exhaust gas discharged from the diesel engine, conventionally, expensive precious metals such as platinum (Pt) or rhodium (Rh) have been used as a catalytically active component in many cases. However, these precious metals are very expensive due to a small amount of reserves and suffer sharp fluctuations in price depending on changes in demand. Therefore, a catalyst, in which the expensive precious metals are not used or precious metal usage is reduced using other metals different from the precious metals, has been actively developed.
For example, Patent Document 1 (JP 2011-140011 A) discloses a CO oxidation catalyst which is obtained in such a manner that Pd is supported on CeO2 carrier particles and a heat treatment is carried out at a temperature in the range of 850 to 950° C. under an oxidizing atmosphere, the CO oxidation catalyst exhibiting CO oxidation activity at a wide temperature range including a low temperature.
Patent Document 2 (JP 2008-156130 A) discloses a catalyst for exhaust gas purification which is obtained by supporting a delafossite-type oxide of 3R type represented by a general formula ABOx (wherein A represents at least one selected from the group consisting of Cu, Ag, Pd, and Pt; and B represents at least one selected from the group consisting of Al, Cr, Ga, Fe, Mn, Co, Rh, Ni, In, La, Nd, Sm, Eu, Y, and Ti) on a carrier made of ceramics or metallic materials, the catalyst for exhaust gas purification having high oxygen storage capacity from a low temperature range to a high temperature range without requiring the presence of a precious metal.
Patent Document 3 (JP 9-225267 A) discloses a catalyst which is obtained using a spinel-type oxide, the catalyst trapping HC at 200° C. or lower and being used in NOx purification by reduction reaction or the like at a high temperature equal to or higher than 200° C.